This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Television broadcast systems throughout the world have migrated from the delivery of analog audio and video signals to modern digital communications systems. For example, in the United States, the Advanced Television Standards Committee (ATSC) has developed a standard called “ATSC Standard: Digital Television Standard A/53” (the A53 standard). The A53 standard defines how data for digital television broadcasts should be encoded and decoded. In addition, the U.S. Federal Communications Commission (FCC) has allocated portions of the electromagnetic spectrum for television broadcasts. The FCC assigns a contiguous 6 MHz channel within the allocated portion to a broadcaster for transmission of terrestrial (i.e., not cable or satellite) digital television broadcasts. Each 6 MHz channel has a channel capacity of approximately 19 Mb/second based on the encoding and modulation format in the A53 standard. Furthermore, the FCC has mandated that transmissions of terrestrial digital television data through the 6 MHz channel must comply with the A53 standard.
Digital broadcast signal transmission standards, such as the A53 standard, define how source data (e.g., digital audio and video data) should be processed and modulated into a signal that is transmitted through the channel. The processing adds redundant information to the source data so that a receiver that receives the signal from the channel may recover the source data even if the channel adds noise and multi-path interference to the transmitted signal. The redundant information added to the source data reduces the effective data rate at which the source data is transmitted but increases the potential for successful recovery of the source data from the transmitted signal.
The A53 standard development process was focused on high definition television (HDTV) and fixed reception. The system was designed to maximize video bit rate for the large high resolution television screens that were already beginning to enter the market. Transmissions broadcast under the ATSC A/53 standard, however, present difficulties for mobile receivers. Enhancements to the standard are required for robust reception of digital television signals by mobile devices.
The FCC requires that broadcasters must use the A53 standard to encode data generated for broadcast transmission. If the transmission of a digital television program broadcast does not require the entire 19 Mb/second capacity of the allocated channel, the broadcaster may use any excess capacity to broadcast other services, possibly even to devices such as portable receivers and cellular telephones. However, the FCC requires that any data transmitted to such other devices using the excess capacity must be transmitted in accordance with the A53 standard. Revision of the A53 standard is possible and is contemplated by the ATSC, however the evolution must occur such that that existing, or so-called legacy, digital television receivers may continue to be used. Similarly, encoding and transmission of signals in accordance with the existing A53 standard may be referred to as legacy encoding and transmission.
Recognizing this fact, in 2007, the ATSC announced the launch of a process to develop a standard that would enable broadcasters to deliver television content and data to mobile and handheld devices via their digital broadcast signal. The resulting standard proposal, referred to as ATSC M/H, is intended to be backwards compatible with the existing A53 standard, allowing operation of existing ATSC services in the same radio frequency (RF) channel without an adverse impact on existing receiving equipment.
The existing or legacy A53 standard, at present, defines generating and transmitting a signal for the intended use by receivers that are generally fixed (e.g., in a home) and that are coupled to large antennas for capturing the transmitted signal. However, the legacy A53 standard transmitted signals are not sufficiently rugged or robust to allow a mobile receiver or a receiver with a small antenna that is used in portable televisions, vehicular televisions, cellular telephones, personal data assistants, etc. to effectively extract the source data encoded in such signals. In particular, the redundancy provided by the combination of Reed Solomon encoder and 2/3-rate trellis used in the transmission of existing A53 standard broadcast signals is not sufficient and lower rate encoders (i.e., those that have greater redundancy) are necessary for mobile applications. Therefore it is desirable to introduce more robust encoding processes adapted to better perform with advanced receivers in mobile, handheld and pedestrian devices. It is further desirable to allow for variable operation within the mobile broadcast including variable code rates and variable data segment sizes for transmission. Additionally, any new system operating within the legacy A53 standard is expected to be backward compatible. As a result, it is also desirable for mobile systems using long data segment transmission to include a mechanism to prevent degradation of performance of the receivers intended for receiving legacy A53 transmission.